Segment Information - Segment Financial Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total sales	$ 2,012.3	$ 1,737.4	$ 3,842.5	$ 3,667.3
Income (loss) from operations	203.3	159.4	394.9	376.7
Interest expense and other, net	(34.6)	(45.1)	(68.0)	(113.2)
Income (loss) before income taxes	168.7	114.3	326.9	263.5
Freight Segment
Segment Reporting Information [Line Items]
Total sales	1,335.9	1,204.7	2,519.2	2,505.7
Transit Segment
Segment Reporting Information [Line Items]
Total sales	676.4	532.7	1,323.3	1,161.6
Operating Segments | Freight Segment
Segment Reporting Information [Line Items]
Total sales	1,347.7	1,217.6	2,543.4	2,531.1
Income (loss) from operations	173.2	141.5	315.0	303.2
Interest expense and other, net	0.0	0.0	0.0	0.0
Income (loss) before income taxes	173.2	141.5	315.0	303.2
Operating Segments | Transit Segment
Segment Reporting Information [Line Items]
Total sales	683.4	538.8	1,340.0	1,176.8
Income (loss) from operations	45.3	40.2	115.4	108.8
Interest expense and other, net	0.0	0.0	0.0	0.0
Income (loss) before income taxes	45.3	40.2	115.4	108.8
Corporate Activities and Elimination
Segment Reporting Information [Line Items]
Total sales	(18.8)	(19.0)	(40.9)	(40.6)
Income (loss) from operations	(15.2)	(22.3)	(35.5)	(35.3)
Interest expense and other, net	(34.6)	(45.1)	(68.0)	(113.2)
Income (loss) before income taxes	(49.8)	(67.4)	(103.5)	(148.5)
Corporate Activities and Elimination | Freight Segment
Segment Reporting Information [Line Items]
Total sales	11.8	12.9	24.2	25.4
Corporate Activities and Elimination | Transit Segment
Segment Reporting Information [Line Items]
Total sales	$ 7.0	$ 6.1	$ 16.7	$ 15.2